Loran-C, an acronym for Long Range Navigation, is a low frequency (100 kHz) system originally intended to provide navigation capability to marine craft operating in the coastal confluence of the continental United States and on the Great Lakes. Because of the low frequency of the Loran-C transmitters and their very high output power (0.5 million watts), the signals also cover over three-quarters of the land area of the United States and, more importantly, more than 90 percent of the population living in metropolitan areas. Thus, Loran-C also is being employed in the avionics market where it is used as a navigational aid by fixed-wing aircraft and helicopters.
In marine applications, a Loran-C receiver on a vessel makes the measurements of the Loran-C signals transmitted from at least three geographic locations. By applying standard trilateration techniques, combined with a knowledge of the location of the relevant transmitters, the Loran-C receiver can compute values to determine the latitude and longitude of the vessel.
In avionics applications, the computation of location is almost identical.
Over the sea and high in the air, Loran-C signals propagate freely. However, land-mobile applications have two significant differences.
The need for this invention arose from a problem in that the structures, including bridges, normally found in a metropolitan environment tend to cause rapid amplitude and phase variations (fading) in the Loran-C signals. When the signal to noise ratio is quite high, a rapid phase variation and a rapid amplitude reduction will often cause cycle slippage. That is, the phase locked loop will attempt to track the phase excursion and may slip to an adjacent cycle, instead. Accordingly, location errors ensue.
This result is a characteristic of phase locked loops when used in a hard limited receiver. Specifically, the phase locked loop bandwidth increases with increasing signal-to-noise.
The instant invention solves the problem by decreasing the phase locked loop (PLL) bandwidth to degrade the ability of the PLL to track rapid phase excursions.
This invention represents a significant advance over the prior art and over this technical field by providing a Loran-C cycle slip reduction technique that consists of decreasing the phase locked loop (PLL) bandwidth to degrade the ability of the PLL to track rapid phase excursions.